1. Field of the Invention
The present invention relates generally to speed change gear systems for automotive vehicles, and more particularly to a speed change gear system suited for applications in which fluid couplings or hydraulic torque converters are to be employed for operation with the gear system, and which is capable of shifting through four forward speeds including an overdrive shift range and one reverse shift range.
2. Description of the Prior Art
As it is usually the practice in the prior art, the reduction gear ratio in the highest gear shift range provided by the speed change gear system is set to 1.0 permitting the input and output shafts directly to be coupled.
In order to meet the requirements set forth by the existing regulations on the vehicle exhaust gases or in order to improve the fuel economy, it is desirable when the vehicle is running at constant and stable speeds with less engine load variations to provide a reduction gear range of less than 1.0 or overdrive range, i.e., a gear ratio with an enhanced revolution speed, thereby preventing the engine speed from deviating from its predetermined revolution range. Therefore, it has been suggested that the speed change gear system be capable of providing an overdrive shift range. It is possible to provide three or four forward speed shift ranges and one reverse shift range employing three planetary gear sets whose arrangement is disclosed in U.S. Pat. No. 3,956,946.
An arrangement including a combination of three planetary gear sets whose members (gears and carriers) constitute five rotationally connected, integrally rotatable units ["integral unit(s)" hereinafter] composing a gear-engaging combination ["gear train" hereinafter] of five integral units with freedom of rotation. A driving power train with a predetermined change gear ratio is formed by the following: one of the five integral units being connected with the output shaft, two of the remaining sets being connected to the input shaft, and the other set being connected to a stationary (locking) member such as a gear casing (however, to be connected to the input shaft, if direct coupling is desired). This gear train permits a switching from one power train to another different power train with a different predetermined reduction gear ratio by switching a mode of connection of five integral units. Thus a set of shift ranges including from 3 to 4 forward shift ranges and one reverse shift range may be realized.
On the other hand, there are possible various modes in connecting and combining the elements of three planetary gear sets which permit numbers of gear trains.
However, the speed change gear system for specific use with the automatic transmissions on vehicles which provides four forward speeds (shift ranges) and one reverse shift range including an overdrive shift range should desirably satisfy the following requirements:
1. The output shaft should always remain coupled to the same member of the gear train regardless of switching from one power train to another;
2. In order to reduce the peripheral speed of the bearings or friction contact portions such as clutches and brakes, and avoid any troubles due to centrifugal force, each individual element within the planetary gear sets should not have an abnormal number of revolutions;
3. The load on gear teeth, i.e., the tangential force, of each element should be small with respect to the strength of the gear teeth;
4. The number of teeth on individual gears of the planetary gear sets should be determined to satisfy the meshing requirements, and smaller-diameter gears (sun gears, planetary pinion gears) should not have less teeth than normally required;
5. The planetary gear sets should be capable of being combined as desired in such a manner that connections among the individual elements are made as short and simple as possible and the connections allow clutches and brakes to be installed with ease;
6. The shifting during the vehicle forward running is done by switching frictional means such as clutches and brakes to select a desired power train. Then, the switching should be effected by switching only one frictional means so that any impact on switching can be reduced to a minimum; and
7. The speed change gear system should be capable of being completed by the existing frictional elements for providing the conventional shifts without requiring additional frictional elements for providing an overdrive gear shift in the existing four forward speed ranges, and yet the system should be capable of providing a reduction gear range of from 0.6 to 0.85.
The known speed change gear systems of the type disclosed herein cannot meet the requirements hereinabove mentioned.
It is therefore one object of the present invention to provide a novel and improved speed change gear system which can meet the above requirements.
It is another object of the present invention to provide a speed change gear system which employs a minimum number of friction elements, i.e., 2 clutches and three brakes to provide four forward speeds including an overdrive shift range, and one reverse employing three planetary gear sets .